1. Field
The present disclosure relates generally to electronics, and more specifically, but not exclusively, to methods and apparatus for adjusting an estimated forward link signal to interference and noise ratio.
2. Background
A modern wireless communication system is required to support a variety of applications and allow multiple users to share a common communications medium. Numerous multiple-access techniques are known in the art, such as Code Division Multiple Access (CDMA). The multi-access concept is based on channel allocation that separates communications among multiple users. By way of example, in a CDMA communication system, channel separation can be achieved by transmitting each signal with a different code that modulates a carrier, and thereby spreads the signal over the entire frequency spectrum. A receiver separates the transmitted signals with a demodulator using a corresponding code to de-spread the desired signal. The undesired signals, whose codes do not match, contribute only to noise.
Using CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” issued Feb. 3, 1990, as well as in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” issued Apr. 7, 1992. Another CDMA system is disclosed in U.S. Pat. No. 6,574,211, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” issued Jun. 3, 2003 (the HDR system). These patents are incorporated herein by reference in their entireties.
A CDMA communications system is typically designed to conform to one or more standards. Such standards include the TIA-856 High Rate Data Air Interface Specification issued by the Telecommunications Industry Association (TIA), which is also known as the “cdma2000 1xEV-DO standard.” The TIA-856 standard is incorporated herein by reference in its entirety.
In EV-DO compatible systems, a basestation (i.e., access point) distributes forward link (FL) control channel power among users (i.e., access terminals), so that each user's service meets a minimum performance requirement. To efficiently distribute the control channel power, the basestation needs information about the FL's signal to interference and noise ratio (SINR) for each user in order for conventional algorithms to allocate just enough power to maintain a target control channel SINR.
However, in conventional EV-DO systems, the basestation does not explicitly receive FL SINR information from the user. Instead, the basestation receives a data rate control (DRC) message, which is a FL data rate request generated by a rate predictor algorithm at the access terminal. The rate predictor requests data rates at which a target packet error rate (PER) can be maintained from a discrete list, based on then-current channel conditions. Thus, in conventional systems, the basestation only maps a discrete DRC data rate to a very coarse estimate of FL SINR. Generally, the FL SINR estimated by a DRC mapping is lower than the actual FL SINR. This bias is especially noticeable for voice over IP (VOIP) data on 1xEV-DO Rev. A/B systems because it is recommended that DRC length for these systems be 8 slots (equivalently, 8*1.67 ms=13.36 ms) to reduce overhead gain on a reverse link (RL). In this case, the FL SINR estimated from the DRC message can be as much as 10 dB lower than the FL SINR estimated at the access terminal, whenever the user is not stationary. On the other hand, for stationary users this bias is within 3 dB. The worst case bias occurs on slow-fading channels. Similar statistics have been seen in system simulations using the Third Generation Partnership Project 2 (3GPP2) methodology.
As a result of the conservative nature of the FL SINR estimate, power is inefficiently allocated. In the case of users with VOIP traffic, the resulting inefficiency limits user capacity of networks having reverse-link interference cancellation (IC) enabled at the basestation. Accordingly, there are long-felt industry needs for methods and apparatus to adjust the FL SINR estimates in order to improve performance of wireless communication systems.